Saturation prevention of current transformer

1. An apparatus for saturation prevention of a current transformer, comprising: a shunt boost controller having an output coupled to a control node of a power switch circuit, the shunt boost controller configured to provide a control signal to the power switch circuit to cause the power switch to controllably reduce a current flowing from a power source to a load by redirecting at least a portion of the current to a ground; a feedback network coupled to a terminal receiving the current flowing to the load, the feedback network configured to provide a feedback signal to the shunt boost controller based on a direct current (DC) voltage representative of the current flowing to the load and to cause the shunt boost controller to provide the control signal to the power switch circuit to turn on when a magnitude of the feedback signal exceeds a threshold magnitude; a pulse width modulation (PWM) generator, separate from the shunt boost controller, configured to supply a PWM signal to modify the control signal and to cause the power switch circuit to turn on with the control signal at a rate based on a frequency of the PWM signal to reduce an imbalance of the load, wherein the frequency of the PWM signal is greater than that of the feedback signal; a first unidirectional pass circuit configured to couple the output of the shunt boost controller with a control node of the power switch; and a second unidirectional pass circuit configured to pass the PWM signal.

2. The apparatus of claim 1 , wherein the PWM signal has a voltage that is greater than a voltage of the feedback signal when the PWM signal is a logic high; and wherein the PWM signal causes the power switch circuit to turn on when the PWM signal is a logic high.

3. The apparatus of claim 1 , wherein the PWM signal is provided to a feedback node of the feedback network to override the feedback signal provided to the shunt boost controller and to cause the power switch circuit to turn on when a magnitude of the PWM signal exceeds the threshold magnitude.

4. The apparatus of claim 1 , further comprising: a sense terminal for sensing an alternating current (AC) current provided to a rectifier by a transformer, wherein the rectifier provides the current flowing to the load, and wherein the PWM signal has a smaller duty-cycle than that of the sensed AC current.

5. The apparatus of claim 1 , further comprising: a sense terminal for sensing an alternating current (AC) current provided to a rectifier by a transformer, wherein the rectifier provides the current flowing to the load, and wherein the PWM signal has a higher frequency than that of the sensed AC current.

6. The apparatus of claim 1 , wherein the feedback network comprises a resistive divider, wherein the resistive divider comprises a first resistive element and a second resistive element connected in series, wherein an intermediate node between the first resistive element and the second resistive element is coupled to a feedback node of the shunt boost controller.

7. The apparatus of claim 6 , wherein a ratio of the first resistive element to the second resistive element is equivalent to a ratio of 1 to N, where N is a positive integer.

8. The apparatus of claim 1 , wherein the feedback signal has a voltage that is compared to a reference voltage, wherein the shunt boost controller drives a gate drive signal with a first voltage to power on the power switch circuit when the voltage of the feedback signal exceeds the reference voltage, and wherein the shunt boost controller drives the gate drive signal with a second voltage to power off the power switch circuit when the voltage of the feedback signal does not exceed the reference voltage, and wherein the first voltage is greater than the second voltage.

9. The apparatus of claim 1 , further comprising: a current transformer configured to sense alternating current (AC) current on an input line and harvest the sensed AC current from the input line; a bridge rectifier configured to convert the sensed AC current into direct current (DC) current to drive a load based on the DC current; and a sample circuit element coupled to an output of the bridge rectifier, the sample circuit element being configured to convert the DC current into DC voltage for measurement.

10. The apparatus of claim 9 , further comprising: a unidirectional pass circuit coupled between the power switch circuit and the load along a signal path from the bridge rectifier, the unidirectional pass circuit being configured to forward bias the DC current from the bridge rectifier to the load when the power switch circuit is powered off and limit current flow from the load to the power switch circuit when the power switch circuit is turned on.

11. The circuit of claim 1 , wherein the second unidirectional pass circuit is configured to pass the PWM signal to a control node of a power transistor of power switch circuit.

12. A power supply circuit comprising: a current transformer configured to wirelessly capture an AC signal from a conductor passing through the current transformer; a full bridge rectifier configured to receive the AC signal and provide a rectified signal; a first unidirectional pass circuit coupled with the full bridge rectifier and configured to receive the rectified signal and provide a output supply signal of the power supply circuit; a shunt switch configured to selectively shunt current from the rectified signal; a controller having an output coupled to a control node of the shunt switch via a second unidirectional pass circuit comprising a diode; a feedback network coupled output supply signal, the feedback network configured to provide a representation of a voltage of the output supply signal to the controller; a PWM generator configured to provide a PWM signal to modify a control signal of the shunt switch to reduce an imbalance of load on the current transformer; and a third unidirectional pass circuit configured to pass the PWM signal.

13. The power supply circuit of claim 12 , wherein the PWM signal is coupled directly with the representation of the output voltage of the feedback circuit via the third unidirectional pass circuit.

14. The power supply circuit of claim 13 , including an analog-to-digital converter (ADC) coupled to a gate terminal of the shunt switch to sense a voltage on the gate terminal, the ADC coupled to an input of the PWM generator to trigger generation of the PWM signal based on the sensed voltage of the gate terminal.

15. The power supply circuit of claim 14 , wherein the PWM generator generates the PWM signal in a second portion of a cycle of the AC signal when the voltage on the gate terminal is sensed by the ADC to be unsymmetrical in a first portion of the cycle, wherein the first portion precedes the second portion.

16. The power supply circuit of claim 12 , wherein the PWM signal is coupled to a gate terminal of the shunt switch via the third unidirectional pass circuit.

17. The power supply circuit of claim 16 , wherein the control circuit includes a comparator configured to compare the representation to the threshold.

18. The power supply circuit of claim 17 , wherein an output of the comparator is coupled to the control node of the shunt switch.

19. A method of operating a power supply to prevent saturation of a current transformer of the power supply, the method comprising: generating an AC signal at an output of a current transformer; rectifying the AC signal to provide a rectified signal at an output of a rectifier; rectifying output current of the power supply using an unidirectional pass circuit coupled directly with the rectifier; providing a representation of an output voltage of the power supply via a feedback network; generating a PWM signal and comparing the PWM signal to a reference voltage; using a power switch, shunting current of the rectified signal from the output current when the representation is higher than a threshold, regardless of a state of the PWM signal; and using the power switch, shunting current of the rectified signal from the output current when the PWM signal is in a first state and the PWM signal exceeds the reference voltage.